Why was the pachycephalosaurus special?

The plant-eating pachycephalosaurus, which means the thick-headed lizard, gets its long name from its strange head. The pachycephalosaurus had a dome-shaped head that made it look as if it was wearing a tight cap. Its skull was about 25 centimetres (10 inches) thick and housed a tiny brain. The pachycephalosaurus probably used its thick head as a weapon when defending itself from other dinosaurs.